1. Field of the Invention
The present invention relates to current sense circuits and, more particularly, to a current sense circuit that operates over a wide range of currents.
2. Description of the Related Art
A current sense circuit is a circuit that measures the magnitude of a current that flows into a load, such as a device under test (DUT). Current sense circuits are commonly implemented with a sense resistor that lies in series with the load, an operational amplifier that has a non-inverting input and an inverting input that are connected to the opposite ends of the sense resistor, and a processing circuit that is connected to the output of the operational amplifier.
In operation, the operational amplifier amplifies the voltage drop that lies across the sense resistor. Knowing the value of the sense resistor along with the voltage drop across the sense resistor, the processing circuit determines the magnitude of the current that flows through the sense resistor using the well-known equation V=IR or I=V/R.
Current sense circuits that incorporate this basic approach work fine when the range of potential currents is relatively small. However, when the range of potential currents is large, such as from less than 1 μA up to 100 mA, these types of current sense circuits often employ resistor switching circuitry to keep the operational amplifier from saturating when the larger currents are present.
The resistor switching circuitry typically includes a bank of sense resistors, switching circuitry that places one or more of the sense resistors from the bank of sense resistors into the current sense circuit, and control circuitry that determines when one sense resistor needs to be replaced with a different sense resistor.
One drawback to using resistor switching circuitry is that there can be a momentary break or dip in the current as one sense resistor is switched out for another sense resistor. This momentary break or dip in the current can cause some loads to reset or otherwise fail to operate as intended. As a result, there is a need for a current sense circuit that can operate over a wide range of currents without any momentary break or dip in the current.